A certain portion of the area below the ceiling is enclosed by me with wooden doors and the lower part for material storage, the wall is available on 3 sides and ceiling above. The enclosed area painted by washable distemper and is adjacent to the bathroom of the flat above. A few months back, I had observed stains which appeared oily in the portion all over the walls and a small portion of the ceiling in the enclosed space. I applied 3 coats over 3 days (1 coat per day) of washable distemper to cover the stains as one coat was insufficient. At that time I tried to diagnose the problem by making 3- 4 holes in the walls expecting some dampness / water seepage, etc. But no visible signs of the same. I checked the bathroom above and there too no visible defects. A plumber and mason were brought in and they said that this could be result of some discharge from the sand used in plaster. Now the oily stains are again gradually showing up.
Can you help me by telling what can these stains be? What are the cause and how are these to be treated?